It is also well known, notably through document FR-2,884,555, to use the calorific energy conveyed by the exhaust gas of internal-combustion engines, in particular those used for motor vehicles, as the hot source providing heating and vaporization of the fluid flowing through the evaporator.
This allows to improve the energy efficiency of this engine by recovering a large part of the energy lost at the exhaust in order to convert it to an energy that can be used for the motor vehicle through the Rankine cycle circuit.
The selection of this working fluid, which undergoes a succession of liquid/vapour phase transformations, is therefore determining.
In fact, the saturation curve of this fluid has to be optimized according to the temperature of the hot source and of the cold source.
Using an aqueous working fluid in a Rankine cycle circuit therefore affords the advantage of having characteristics allowing to obtain a maximum saturation curve while having the advantage of not being dangerous.
However, water has the specific feature of having a freezing point at low temperatures (around 0° C.) and antifreeze additives such as glycol are usually added thereto in order to lower this freezing point to acceptable temperature levels, of the order of −15° C. to −30° C.
Adding such additives has the drawback of changing the characteristics of water, in particular its vaporization characteristics, and the hot source from the exhaust gas may be insufficient to perform this vaporization in a satisfactory manner.
Furthermore, in the course of time, this additive-containing water undergoes unpredictable aging as the liquid/vapour phase changes take place. This unpredictable aging can lead to incomplete phase changes for this water, which generates a Rankine cycle circuit dysfunction.
The present invention aims to overcome the aforementioned drawbacks by means of a device and of a method that limit or even prevent freezing of the working fluid without causing changes in the liquid/vapour phase transformation characteristics.